This invention relates to detection of acceleration, such as by an accelerometer, and particularly to an accelerometer which is suitable for use a car body control system for a motor vehicle. In such a an accelerometer capable of accurately detecting acceleration in the range of 0.about..+-.1(G) and 0 .about.10(Hz) is required, . being equal to 9.8(m/s.sup.2).
A large number of have been developed for use as an accelerometer in which a is displaced in response to acceleration and the displacement is detected by a detecting system. Known detecting systems include a piezoelectric system using the piezoelectric effect of a piezoelectric material, a strain gauge system using piezoresistance effect, a servo system having a force feedback system, a magnetic system utilizing a differential transformer, an optical system utilizing a photo-interrupter, a capacitance system utilizing a detector produced by a technique miniaturization etching of silicon, and so forth.
Among these systems, it has been proposed to drive the capacitance type sensor of an electrostatic servo system. The publication TRANSDUCERS '87, The 4th International Conference on Solid-State and Actuators, Pages 395 to 398, and U.S. Pat. No. 4,483,194 are cited as showing examples of such a device.
The system which drives the capacitance type sensor utilizing the conventional miniaturization etching of silicon by means by an electrostatic servo system involves the drawbacks that a compensation circuit for linearizing non-linearity in the system is needed because non-linearity of the electrostatic servo mechanism is great, the output characteristics can not be adjusted easily and the production yield of the sensor is low.
The typical structure of a capacitance type accelerometer includes the electrodes 2 to 4 and cantilever 1 as generally shown in FIG. 2(b), and an equivalent electrical circuit thereof is shown in FIG. 2(a). A movable electrode 2 serving also as a weight is formed at the tip of a cantilever 1 by etching a silicon plate 6 from both of its surfaces. Fixed electrodes 3, 4 are made of a metal, such as aluminum, and are formed by vapor deposition onto glass plates 7, 8, respectively.
Assuming that the capacitances between the movable electrode 2 and the fixed electrodes 3 and 4 are C.sub.1, C.sub.2, respectively, as seen in FIG. 2(a), the values of C.sub.1, C.sub.2 are proportional to the displacement of the movable electrode 2, that is, the acceleration .alpha.(G). The most typical measurement method of the conventional capacitance type accelerometer operates to detect the acceleration .alpha.(G) from the absolute values of the capacitances C.sub.1, C.sub.2 or their difference .DELTA.C. As will be describe next, this measurement method involves the problem that the output characteristics fluctuate greatly due to the variance of the initial gap dimension between the movable electrode 2 and the fixed electrodes 3, 4 that occurs during production.
FIG. 3 shows an example of the relation between the displacement of the movable electrode 2 and the capacitance C.sub.1, C.sub.2 (PF) and the difference .DELTA.C when the movable electrode 2 undergoes a displacement .omega. in accordance with the acceleration .alpha.(G). The diagram shows the case where, with the movable electrode 2 at its neutral point, the initial gap d.sub.o between the movable electrode 2 and the fixed electrodes 3, 4 is 3 .mu.m. The displacement in the positive direction represents the state of the movable electrode 2 being moved in the upper direction (toward the fixed electrode 3 side), and the displacement in the negative direction represents the state of the movable electrode 2 being moved in the lower direction (toward the fixed electrode 4 side). As apparent from this graph, the closer the movable electrode 2 moves toward the fixed electrode 3, the larger the static electric capacitance C.sub.1 will become; and, on the other hand, the closer the movable electrode 2 moves toward the fixed electrode 4, the larger the static electric capacitance C.sub.2 will become. And also, the difference .DELTA.C of the static electric capacitance C.sub.1 -C.sub.2 will correspondingly increase respectively in the positive direction and in the negative direction from the ZERO neutral point (reference position) with displacement of the movable electrode 2 in the respective directions. Further, in case the movable electrode 2 stands at the neutral point, both of the static electric capacitances C.sub.1, C.sub.2 have the same value (about 6.5 pF). Thus, it is possible to determine the displacement of the movable electrode 2 by detecting any one of C.sub.1, C.sub.2 and .DELTA.C.
As shown in FIG. 3, the capacitances C.sub.1, C.sub.2 and their difference .DELTA.C exhibit a large non-linearity with respect to the displacement .omega. of the movable electrode 2 (which is proportional to the acceleration .alpha.) and so it is difficult to detect with high accuracy the acceleration .alpha. from such values. Incidentally, the capacitance C between the electrodes is given by the following formula, as is well known: ##EQU1## where .epsilon.o: vacuum dielectric constant,
s: area of electrode, PA1 d: gap between electrodes.
Since the capacitance C is inversely proportional to the gap dimension d, the non-linearity between acceleration .alpha. and values of C.sub.1, C.sub.2 and .DELTA.C becomes great as shown in FIG. 3. As can be understood from the formula (1) and FIG. 3, if the initial gap d.sub.o between the movable electrode 2 and the fixed electrodes 3, 4 is greater than 3 .mu.m when the production is complete, the sensitivity and non-linearity of the capacitance C.sub.1, C.sub.2 and their difference .DELTA.C with respect to the change of the acceleration .alpha. will fluctuate greatly. Therefore, when the acceleration .alpha. is detected from the change in capacitance .DELTA.C, the variance of the initial gap dimension at the time of production must be an extremely small value. In practice, however, this conventional device has the drawback that the initial gap dimension at the time of production is not constant.
In addition to the problem of non-linearity inherent in the accelerometer device, as discussed above, the capacitance type acceleration sensor also is subject to problems relating to the construction and operation of the servo control systems used therewith. FIG. 1(b) shows an example of a known capacitance type acceleration sensor and an associated servo control system.
In FIG. 1(b), a moving electrode 130 is supported by a silicon beam 133, and this moving electrode 130 is displaced in response to acceleration. Each of the fixed electrodes 131a, 131b for detecting the displacement of the movable electrode 130 and each of the fixed electrodes 132a, 132b for static electric servo-operation are opposed, with the movable electrode 130 being interposed between them. The movable electrode 130 is held to be ZERO potential.
A capacitance detector circuit 134 employs an A/C bridge for instance, and this detector circuit detects the difference .DELTA.C between the static capacitance C.sub.1 between the fixed electrode 131a and the movable electrode 130 and the static capacitance C.sub.2 between the fixed electrode 131b and the movable electrode 130 (.DELTA.C presents the displacement of the movable electrode 130), and this measure of .DELTA.C is output via an amplifier 135 as the voltage value Va. This output value Vout (Va) becomes the sensor output, and the acceleration is detected on the basis of this sensor output.
Also, a static electric servo-operation is performed on the basis of the voltage value Va so that the movable electrode 130 is restricted to the reference position (the neutral point), in other works, so that .DELTA.C becomes ZERO. More particularly, for example Vb+Va' is applied to one side of the fixed electrodes 132a, 132b for static electric servo-operation, and Vb-Va' is applied to the other side thereof, via a root circuit 136 and a voltage applying circuit 137, and the static electric force (attractive force) necessary for the static electric servo-operation is generated at each of the fixed electrodes 132a, 132b.
The root circuit 136 plays the role of a compensating circuit to compensate for the non-linearity of the static electric servo-operation mechanism. Namely, since the static electric force F generated at the fixed electrodes 132a, 132b is proportional to the square of the applied voltage, when no consideration is given to variations in the applied voltage, the inherent non-linearity of the static electric servo-operation mechanism will be made even larger. Accordingly, it is necessary to provide a compensating circuit 136 to compensate for said non-linearity of the static electric servo-operation mechanism.
That is, the static electric force F is expressed by the following equation. ##EQU2## (here, .epsilon. is a dielectric constant, S is the area of the electrode; V is an applied voltage; and D is a duty of the applied voltage; in the conventional case, D=1, since the magnitude of the applied voltage is varied.) EQU V.varies..sqroot.F (3)
And therefore, it is necessary to take the root of the voltage V applied to the fixed electrodes so as to compensate for the non-linearity of the static electric servo-operation mechanism.
In the case of the static capacitance type acceleration sensor which is driven by the static electric servo driving system described above, when the root circuit is used as a compensating circuit to compensate for the non-linearity of the static electric servo-operation mechanism, since the price of this circuit itself is expensive, the cost of the whole apparatus increases accordingly. Further, in the case of adjusting the output characteristics without utilizing such a root circuit, the cost of the adjusting increases because of the difficulty in accomplishing the adjusting, and also, the yield rate of the product becomes worse.
Further, in the case of this kind of static capacitance type acceleration sensor which is driven by a static electric servo driving system, since it is necessary to individually and separately provide the fixed electrodes for detecting capacitance and the fixed electrodes for the static electric servo-operation, the area of the electrodes is increased as well as the number of components, and thus the size of the apparatus becomes larger and the cost thereof also increases.